Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and nitrogen oxides (“NOx”), but also condensed phase materials (liquids and solids) which constitute the so-called particulates or particulate matter. Often, catalyst compositions and substrates on which the compositions are disposed are provided in diesel engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For example, diesel exhaust systems can contain one or more of a diesel oxidation catalyst, a soot filter and a catalyst for the reduction of NOx.
Oxidation catalysts that contain platinum group metals, base metals and combinations thereof, are known to facilitate the treatment of diesel engine exhaust by promoting the conversion of both HC and CO gaseous pollutants and some proportion of the particulate matter through oxidation of these pollutants to carbon dioxide and water. Such catalysts have generally been contained in units called diesel oxidation catalysts (DOCs), which are placed in the exhaust of diesel engines to treat the exhaust before it vents to the atmosphere. In addition to the conversions of gaseous HC, CO and particulate matter, oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) also promote the oxidation of nitric oxide (NO) to NO2. The total particulate matter emissions of diesel exhaust are comprised of three main components. One component is the solid, dry, solid carbonaceous fraction or soot fraction. This dry carbonaceous matter contributes to the visible soot emissions commonly associated with diesel exhaust. A second component of the particulate matter is the soluble organic fraction (“SOF”). The SOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust. It is generally present as condensed liquids at the standard particulate collection temperature of 52° C. in diluted exhaust, as prescribed by a standard measurement test, such as the U.S. Heavy Duty Transient Federal Test Procedure. These liquids arise from two sources: (1) lubricating oil swept from the cylinder walls of the engine each time the pistons go up and down; and (2) unburned or partially burned diesel fuel.
The third component of the particulate matter is the so-called sulfate fraction. The sulfate fraction is formed from small quantities of sulfur components present in the diesel fuel. Small proportions of SO3 are formed during combustion of the diesel, which in turn combines rapidly with water in the exhaust to form sulfuric acid. The sulfuric acid collects as a condensed phase with the particulates as an aerosol, or is adsorbed onto the other particulate components, and thereby adds to the mass of TPM.
As emissions regulations become more stringent, there is a continuing goal to develop diesel oxidation catalyst (DOC) systems that provide improved performance, for example, light-off performance. There is also a goal to utilize components of DOCs, for example, the zeolites and palladium, as efficiently as possible.